Method for introducing a moisture-impermeable layer into the ground, as well as a trench obtained by such a method

ABSTRACT

A method for introducing a moisture-impermeable layer into the ground, wherein the moisture-impermeable layer is placed into the ground in a trench, which trench is supported by one or more casing members and filled with a clay-containing mixture of dry solids, which mixture can form a moisture-resistant gel, comprising powdery or pulverized smectite and/or smectite-containing natural rock and a water-soluble polymer.

[0001] Method for introducing a moisture-impermeable layer into the ground, as well as a trench obtained by such a method.

[0002] The present invention relates to a method for introducing a moisture-impermeable layer into the ground, wherein the moisture-impermeable layer is placed into the ground in a trench, as well as a trench obtained by such a method.

[0003] German Offenlegungsschrift no. 37 17 885 discloses a method for producing a sealing course of mineral sealing materials, such as, for example, bentonite, using a trench method, wherein the dry material is introduced into a trench. One drawback of said method is the fact that the dry mixture comes into direct contact with possibly contaminated water from the environment upon being introduced into the trench, which interferes with the swelling capacity of the bentonite, which is still dry at that stage, as a result of which the durability thereof is affected in comparison with bentonite that has been hydrated in advance. No further details with regard to the composition are provided.

[0004] U.S. Pat. No. 3,986,365 relates to a method for forming a moisture-impermeable layer, wherein a composition of sand and bentonite is mixed with a water-soluble polymer. The mixture thus obtained is only suitable for horizontal use under water basins, and owing to the properties of bentonite and sand, which are not specified in detail, this mixture is not suitable for use in vertical sealing systems.

[0005] Such a method is known per se from U.S. Pat. No. 5,259,705. According to said patent a moisture-impermeable layer is introduced into the ground, using a special installation method, which employs guide containers. Such guide containers consist of vertical surfaces, which are interconnected by means of spacers. After the guide containers have been installed in the ground, a fluid mixture is introduced into the guide containers, thus creating a barrier wall in the ground.

[0006] From U.S. Pat. No. 4,664,560 there is also known a method for installing sheet piles in the ground, which method comprises the placing into the ground of vertical barrier walls, which are interconnected. Such barrier walls are subject to rust formation and/or decaying processes, however, so that a durable screen is not obtained. Moreover, the joints between such barrier walls are susceptible to leakage, which interferes with obtaining the intended effect, in particular the provision of a barrier.

[0007] From U.S. Pat. No. 4,877,357 there is furthermore known a method for introducing a moisture-impermeable layer into the ground, wherein a trench is dug in the ground, which trench is subsequently filled with a fluid, which fluid will eventually form a water-impermeable layer. Such a method can be considered to be a slurry method, wherein slurries with a base of cement and bentonite mixed with water are used, which slurries are liquid at the moment of being processed, owing to the addition of retardants. The cement that is present in such slurries will cause the slurry to harden into a solid material after some time. Such methods based on liquid slurries are characterized by a very low density in dry condition and a large specific surface area, however, as a result of which cracks are easily formed due to contraction. In addition it has become apparent that the slurry only provides limited resistance against penetration of the adjacent soil, so that local failure of the wall may occur. Since the impermeability of the slurries is relatively high in comparison with the density of the materials having a higher dry mass that are used in conventional horizontal seals, a relatively thick wall is necessary in order to provide sufficient certainty.

[0008] The object of the present invention is to provide a method for introducing a moisture-impermeable layer into the ground, wherein the aforesaid problems of the prior art are eliminated to a significant degree.

[0009] Another object of the present invention is to provide a method for introducing a moisture-impermeable layer into the ground, which method comprises a sealing system that corresponds to the horizontal sealing systems or the surface sealing systems that have been usual so far.

[0010] The method as referred to in the introduction is according to the present invention characterized in that the trench is supported by one or more casing members, which trench is subsequently filled with a clay-containing mixture of dry solids, which mixture can form a moisture-resistant gel, comprising powdery or pulverized smectite and/or smectite-containing natural rock and a water-soluble polymer.

[0011] When said dry mixture comes into contact with water or with a water-containing fluid, which takes place in the case of rainfall or of water or an aqueous solution seeping through, gelation will take place spontaneously and will continue for some time, resulting in the formation of a moisture-resistant, water-impermeable gel. The use of a dry mixture leads to a proper filling degree or densification, resulting in satisfactory crosslinking upon contact with water.

[0012] Such a clay-containing mixture of dry solids is known per se from European patent no. 0 682 684 in the name of the present applicants. From said European patent the special application in accordance with the present invention, viz. the forming of a trench in the ground, which trench is subsequently filled with the clay-containing mixture of dry solids, is not known, however, which application must be considered to be novel and inventive, therefore. In addition to that, a clay-containing mixture capable of forming a moisture-resistant gel is known from European patent no. 0 335 653, but the application that is known therefrom is essentially different from the present application.

[0013] In the present invention it is in particular preferable to obtain the trench in the ground by using one or more casing members, wherein one or more hollow elements are installed in the ground, said elements consisting of at least two plates which are interconnected by means of spacers, which plates may include a disconnectable closure part on their bottom sides. A tight seal on the sides of the elements may also function as a spacer, which seal can also serve as a guide for the adjacent elements.

[0014] In such an embodiment it is possible that the hollow element is filled with the clay-containing mixture of dry solids first before being installed in the ground, after which the element thus filled is installed in the ground.

[0015] According to a special embodiment of the present invention it is also possible, on the other hand, to install the hollow element in the ground first and then fill the element with the clay-containing mixture of dry solids.

[0016] In order to effect an adequate densification of the clay-containing mixture of dry solids, it is preferable to remove the aforesaid element from the trench by means of vibration, after which the densified, clay-containing mixture of dry solids thus remains behind in the trench. If a hollow element including a disconnectable closure part at the bottom side of the plates is used, the closure part will remain behind in the trench after the hollow element has been removed from the trench. Such a closure part prevents soil and/or water from penetrating into the hollow element when said element is installed in the ground.

[0017] In order to obtain an adequate densification of the clay-containing mixture of dry solids it is in particular desirable in specific embodiments that such densification takes place by causing the element that is present in the trench to vibrate, as a result of which the mixture in the trench is densified as a result of the transmission of vibration energy.

[0018] In a specific embodiment it is preferable the hollow element is lined with a material on the inside before the element is filled with the clay-containing mixture of dry solids, which material prevents the clay-containing mixture of dry solids from adhering to the hollow element.

[0019] Such a material facilitates sliding of the hollow element upon being removed from the trench, and it also functions as a barrier layer between the clay-containing mixture of dry solids and the adjacent soil. An example of such a material is geofabric.

[0020] In specific embodiments of the present invention it is preferable to use one or more hollow elements comprising a number of compartments. Thus it is possible to obtain a different composition, for example one compartment containing the clay-containing mixture of dry solids and one compartment containing sand. Such an embodiment is in particular advantageous if it is not necessary to fill the trench with the same clay-containing mixture of dry solids along its entire length, depth and width, so that generally a cost saving is achieved.

[0021] In a special embodiment of the present method the trench that has been formed in the ground is lined on both longitudinal sides thereof with one or more casing members consisting of one or more panels, after which the space thus enclosed by the panels is filled with the clay-containing mixture of dry solids. In order to effect an adequate densification of the clay-containing mixture of dry solids that is present in the trench it is thus possible to vibrate the panels that are present in the trench. It is in particular desirable, however, for the clay-containing mixture of dry solids in the trench to be densified by removing the panels from the trench in such a manner that this leads to densification of the clay-containing mixture of dry solids that is present in the trench. Such densification is obtained in particular if the panels are vibrated upon being removed from the trench.

[0022] In a special embodiment it is preferable to line the panels with a material on the side facing away from the soil before the panels are filled with the clay-containing mixture of dry solids, which material prevents the clay-containing mixture of dry solids from adhering to the panel. Such a material makes it easier for the panel to slide upon being removed from the trench, and it furthermore functions as a barrier layer between the clay-containing mixture of dry solids and the adjacent soil.

[0023] Furthermore it is possible to install a mat consisting of two geofabric cloths, between which the clay-containing mixture of dry solids is fixed, in the trench as the casing member.

[0024] It should be understood that it is possible to use a number of, possibly interconnected or overlapping, hollow elements, mats and/or panels disposed one behind another in order to obtain a trench filled with the clay-containing mixture of dry solids, depending on the dimensions of the trench and the method used thereby, viz. the hollow elements, the mats and/or the panels installed on either longitudinal side of the trench.

[0025] Since the densification of the clay-containing mixture in the trench has resulted in a certain drop in the level of said mixture in the trench, it is preferable to fill the space that has thus formed, preferably with the same clay-containing mixture of dry solids. It is not inconceivable, however, to use a filling material other than the present clay-containing mixture of dry solids, for example sand. Thus the trench is filled in stages.

[0026] According to a special embodiment of the present invention it is possible to use a supporting fluid when forming the trench in the ground, in order to prevent the side walls of the trench from caving in. Furthermore it is preferred that the supporting fluid be forced out of the trench upon installation of the hollow elements and/or the mat in the trench.

[0027] In order to obtain a gel that is sufficiently resistant against moisture, the smectite is preferably in an inactive state, that is, the lattice sodium and lithium ions that may be present do not make up more than 30% of the sum of the exchangeable lattice cations. The smectite may be one of the following: montmorillonite, beidellite, hectorite, montronite, saponite, illite, allevardite, mixtures thereof, natural rock containing one or more of the above (such as bentonite), or artificial mixtures of smectite-type silicate compounds (for example Laponite (trademark), Laporte Co., Great Britain).

[0028] The moisture content of the present clay-containing mixture of dry solids is preferably <20%, whereby the desired moisture content is determined to a significant degree by the manner in which said mixture is processed, as well as by the dimensions of the trench. Thus a moisture content of <10% will be preferred when a narrow trench is to be filled, in connection with the required flow characteristics of the clay-containing mixture; it will also be possible, however, to fill a hollow element outside the trench with a mixture having a moisture content of maximally 20%.

[0029] Preferably, the clay-containing mixture of dry solids additionally comprises a powdery solid activating agent in an amount of 0.8-6.0 wt. %, based on the weight of the smectite. In addition to that the clay-containing mixture preferably comprises more than 0.5 wt. % of one or more solid inert fillers having an average particle size of more than 0.05 mm, based on the weight of the smectite. In a special embodiment the clay-containing mixture of dry solids contains in particular 1-95 wt. % of a solid inert filler, based on the weight of the overall mixture. It is in particular preferred for the solid inert filler to have a particle size of 0.1-8.0 mm.

[0030] Each water-soluble polymer which is capable of reacting with smectite can be used as the polymer component in the present clay-containing mixture of dry solids. Examples of such polymers are polymers that contain —COOH, COO¹Me⁺ (Me⁺ is a monovalent metal-cation), —CONH₂, —OH and/or ═C—O—C═ groups as functional groups. Examples of polymers to be used are polyacrylamide having a degree of hydrolysis of less than 40%, polymethacrylamide, acrylamide-acrylic acid-copolymers, vinyl alcohol-acrylic acid copolymers, polyethylene oxides and mixtures thereof. It is also possible to use graft polymers, such as acrylamide grafted onto a cellulose backbone. The molecular weight of the polymer is generally less than 15,000,000, with the preferred molecular weight ranging between 400,000 and 10,000,000.

[0031] The clay-containing mixture of dry solids may also comprise one or more solid inert fillers, for example sand, silica compounds, pulverized rock or minerals, a pulverized ceramic material or a mixture thereof. 

1. A method for introducing a moisture-impermeable layer into the ground, wherein the moisture-impermeable layer is placed into the ground in a trench, characterized in that the trench is supported by one or more casing members, which trench is subsequently filled with a clay-containing mixture of dry solids, which mixture can form a moisture-resistant gel, comprising powdery or pulverized smectite and/or smectite-containing natural rock and a water-soluble polymer.
 2. A method according to claim 1, characterized in that the trench in the ground is obtained by using one or more casing members, wherein one or more hollow elements are installed in the ground, said elements consisting of at least two plates which are interconnected by means of spacers, which plates may include a disconnectable closure part on their bottom sides.
 3. A method according to claims 1-2, characterized in that said the hollow element is filled with the clay-containing mixture of dry solids first before it is installed in the ground in order to obtain a trench containing a clay-containing mixture of dry solids.
 4. A method according to claims 1-2, characterized in that the hollow element is installed in the ground first, after which it is filled with the clay-containing mixture of dry solids in order to obtain a trench containing a clay-containing mixture of dry solids.
 5. A method according to claims 1-4, characterized in that the element filled with the clay-containing mixture of dry solids is removed from the trench in such a manner that the clay-containing mixture of dry solids that remains behind in the trench is densified.
 6. A method according to claim 5, characterized in that said element is removed from the trench by being vibrated, wherein the closure part that may be present is disconnected and remains behind in the ground.
 7. A method according to claims 1-6, characterized in that first the hollow element is lined with a material on the inside, after which filling with the clay-containing mixture of dry solids takes place, which material prevents the clay-containing mixture of dry solids from adhering to the hollow element.
 8. A method according to claim 1, characterized in that the trench that has been formed in the ground is lined on both longitudinal sides thereof with one or more casing members consisting of one or more panels, after which the space thus enclosed by the panels is filled with the clay-containing mixture of dry solids.
 9. A method according to claim 8, characterized in that the panels are lined with a material on the side facing away from the soil before the panels are filled with the clay-containing mixture of dry solids, which material prevents the clay-containing mixture of dry solids from adhering to the panel.
 10. A method according to claims 8-9, characterized in that after the space enclosed by said panels has been filled with the clay-containing mixture of dry solids, said panels are removed from the trench in such a manner that the clay-containing mixture of dry solids that remains behind in the trench is densified.
 11. A method according to claim 10, characterized in that said panels are removed from the trench by being vibrated.
 12. A method according to claims 1-11, characterized in that the clay-containing mixture of dry solids that is present in the trench is densified by vibrating one or more hollow elements or panels.
 13. A method according to claims 5-6 and 10-12, characterized in that a supplementary amount of clay-containing mixture of dry solids is added to the densified clay-containing mixture of dry solids in the trench.
 14. A method according to claim 1, characterized in that a mat consisting of two geofabric cloths, between which the clay-containing mixture of dry solids is fixed, is installed in the trench as the casing member.
 15. A method according to claims 1-14, characterized in that a supporting fluid is used when forming the trench in the ground, in order to prevent the side walls of the trench from caving in.
 16. A method according to claim 15, characterized in that said supporting fluid be forced out of the trench upon installation of one or more hollow elements and/or mats in the trench.
 17. A method according to claims 1-16, characterized in that the smectite is in an inactive state, that is, the lattice sodium and lithium ions that may be present do not make up more than 30% of the sum of the exchangeable lattice cations.
 18. A method according to claims 1-17, characterized in that the clay-containing mixture of dry solids additionally comprises a powdery solid activating agent in an amount of 0.8-6.0 wt. %, based on the weight of the smectite.
 19. A method according to claims 1-18, characterized in that the clay-containing mixture of dry solids additionally comprises more than 0.5 wt. % of one or more solid inert fillers having an average particle size of more than 0.05 mm, based on the weight of the smectite.
 20. A method according to claims 1-19, characterized in that said solid inert filler is sand, a silica compound, pulverized rock or a mineral, a pulverized ceramic material or a mixture thereof.
 21. A method according to claims 1-20, characterized in that the clay-containing mixture of dry solids contains 1-95 wt. % of a solid inert filler, based on the weight of the overall mixture.
 22. A method according to claims 1-21, characterized in that said solid inert filler has a particle size of 0.1-8.0 mm.
 23. A method according to claims 1-22, characterized in that said clay-containing mixture of dry solids has a moisture content of less than 20%.
 24. A trench obtained in accordance with the method as defined in claims 1-23. 